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This invention relates to fuel cells, and more particularly to the design of a groove profile for use in forming seals between different elements of a conventional fuel cell or fuel stack assembly, to prevent leakage of gases and liquids required for operation of the individual fuel cells. The invention also relates to the formation of such seals with a novel sealing material.
There are various types of known fuel cells. One type of fuel cell currently considered practical for use in many applications is a fuel cell employing a proton exchange membrane (PEM). PEM fuel cells enables simple, compact fuel cells to be designed, which are robust, can be operated at temperatures not too different from ambient temperature, and which do not have complex requirements with respect to fuel, oxidant, and coolant supplies.
A single conventional fuel cell generates a relatively low voltage. In order to provide a useable amount of power, therefore, fuel cells are commonly configured into fuel cell stacks typically containing 10, 20, 30, and even 100xe2x80x2s or more fuel cells in a single stack. While this provides a single unit capable of generating useful amounts of power at usable voltages, the design can be complex, and can include numerous elements all of which must be carefully assembled.
For example, a conventional PEM fuel cell requires two flow field plates, an anode flow field plate, and a cathode flow field plate. A membrane electrode assembly (MEA) including the actual proton exchange membrane is provided between the two flow field plates. Additionally, a gas diffusion media or layer (GDM/GDL) is sandwiched between each flow field plate and the proton exchange membrane. The gas diffusion media enables diffusion of an appropriate gas, either the fuel or the oxidant, to the surface of the proton exchange membrane, while at the same time providing conduction of electricity between the associated flow field plate and the PEM.
This type of a basic cell structure itself requires two seals, with each seal being provided between one of the flow field plates and the PEM. Moreover, the seals have to be of relatively complex configuration. In particular, and as detailed below, the flow field plates for use in the fuel cell stack have to provide a number of functions, and a complex sealing arrangement is therefore required.
For a fuel cell stack, the flow field plates typically provide apertures or openings at either end so that a stack of flow field plates defines elongate channels extending perpendicularly to the flow field plates. As fuel cells require flows of a fuel, an oxidant, and a coolant, this typically requires at least three pairs of ports or six ports in total. This is because it is necessary for the fuel and the oxidant to flow through each fuel cell. A continuous flow is required as it ensures that although most of the fuel or oxidant may be consumed, any contaminants are continually flushed through the fuel cell.
The foregoing assumes that the fuel cell is a compact type of configuration provided with water or the like as a coolant. There are also known stack configurations which use air as a coolant, either relying on natural convection or forced convection. Such fuel cell stacks typically provide open channels through the stacks for the coolant, and therefore the sealing requirements are diminished. Commonly, it is then only necessary to provide sealed supply channels for the oxidant and the fuel.
Consequently, each flow field plate typically has three apertures at each end, and each aperture represents either an inlet or outlet for one of the fuel, oxidant, or coolant. In a completed fuel cell stack, these apertures align to form distribution channels extending through the entire fuel cell stack. It should therefore be appreciated that the sealing requirements are complex and difficult to meet. However, it is possible to have multiple inlets and outlets to the fuel cell for each fluid, depending on the stack/cell design. For example, some fuel cells have two inlet ports for each of the anode, cathode, and coolant, two outlet ports for the coolant, and a single outlet port for each of the cathode and anode. However, other combinations are also possible.
The coolant most commonly flows across the back of each fuel cell so as to flow between adjacent individual fuel cells. This is not essential, however, and as a result, many fuel cell stack designs have cooling channels only at every second, third, or fourth plate. This allows for a more compact stack with thinner plates but such an arrangement may provide less than satisfactory cooling. It also requires another seal between each adjacent pair of individual fuel cells. Thus, in a completed fuel cell stack, each individual fuel cell requires two seals just to seal the membrane exchange assembly to the two flow field plates. A fuel cell stack with 30 individual fuel cells will require 60 seals for that purpose. Additionally, as noted, a seal is required between each adjacent pair of fuel cells and end seals for the current collectors. For a 30 cell stack, therefore, this requires an additional 31 seals, Thus, a 30 cell stack requires a total of 91 seals, excluding the seals for bus bars, current collectors, and endplates, and each of these would be of a complex and more elaborate construction. With additional gaskets required for bus bars, insulator plates, and endplates, the number easily reaches 100 seals of varying configurations in a single 30 cell stack.
These seals can be formed by providing channels or grooves in the flow field plates, and then providing prefabricated gaskets in the channels or grooves to effect a seal. In known configurations, the gaskets and/or the sealing material are specifically polymerized and formulated to resist degradation from contact with various of the materials of construction in the fuel cell, and the various gases and coolants which are aqueous, organic, and inorganic fluids used for heat transfer. This means that assembly technique for a fuel cell stack will be complex, time consuming, and offers many opportunities for error.
Accordingly, in a first technique, a resilient seal can be provided as a floppy gasket seal molded separately from individual elements of the fuel cells by known methods such as injection, transfer or compression molding of an elastomer.
A second technique for providing such resilient seals involves application of an uncured sealing material to the fuel cell plates by dispensing the uncured sealing material, silk screening the uncured sealing material, or spraying uncured sealing material onto the fuel cell plate to a predetermined thickness, and then curing the sealing material to achieve desired elastomeric properties.
A third technique for providing resilient seals involves insert injection molding in which a resilient seal is fabricated on a plate and assembly of the unit is simplified. According to this technique, the gasket is adhered to the fuel cell plate sufficiently to allow its handling and assembly in the fuel cell stack. Such insert injection molded gaskets can be designed with improved groove and seal profiles to optimize the various sealing forces occurring within a fuel cell stack. The basic process for insert injection molding is known in the art, and reference may be had, for example, to U.S. Pat. No. 4,865,793 (September 1989). As noted hereinafter, this is the most preferred technique according to the present invention.
As an additional consideration, formation or manufacture of such seals or gaskets is complex, and there are generally only two known methods of manufacture. For the first technique, the individual floppy gasket seal can be formed by molding it in a suitable mold. This can be relatively complex and expensive, and for each fuel cell configuration, it would require the design and manufacture of a mold corresponding exactly to the shape of the associated grooves in the flow field plates. This does, however, have the advantage that the designer has freedom in choosing the cross-section of each gasket or seal, and it does not have to have a uniform thickness.
Another method would be to cut each gasket from a solid sheet of material. This has the advantage that a cheaper and simpler technique can be used in which it is only necessary to define the shape of the gasket in a plan view, and to manufacture a cutting tool of that configuration. The gasket can then be cut from a sheet of appropriate material of appropriate thickness. The disadvantage is that one can only form gaskets having uniform thickness. Additionally, it leads to considerable waste of materials. For example, for each gasket, a portion of the material corresponding to the area of a flow field plate is used yet the surface area of the seal itself is only a small fraction of the area of the flow field plate.
Fuel cell stacks after assembly are clamped to secure the elements and ensure that adequate compression is applied to the seals and active area of the fuel cell stack. This ensures that the contact resistance is minimized and the electrical resistance of the individual fuel cells are at a minimum. To this end, fuel cell stacks typically have two end plates which are configured to be sufficiently rigid so that their deflection under pressure is within an acceptable tolerance. The fuel cell also has current bus bars to collect and concentrate current from the fuel cell to a small pick up point, where the current is then transferred to the load via conductors. Insulation plates are also used to isolate thermally and electrically the current bus bars and the endplates from one another. A plurality of elongated rods, bolts and the like, are then provided between the pairs of plates so that the fuel cell stack can be clamped together between the plates by tension rods. Rivets, straps, piano wire, metal plates and other mechanisms can also be used in clamping stacks together.
To assemble the stack, the tension rods extend through one of the plates and an insulator plate, bus bars including seals are placed on top of the endplate, and the individual elements of the fuel cell are built up within the space defined by the rods or some other positioning tool. For each fuel cell, this operation requires the steps of (a) placing a seal to separate the fuel cell from the preceding fuel cell; (b) locating a flow field plate on the seal; (c) locating a seal on the first flow field plate; (d) placing a GDM within the seal on the flow field plate; (e) locating a membrane electrode assembly (MEA) on the seal; (f) placing an additional GDM on top of the MEA; (g) preparing a further flow field plate with a seal and placing this arrangement on top of the membrane exchange assembly while at the same time ensuring that the seal of the second plate falls around the second GDM; (h) with the second or upper flow field plate then having a groove for receiving a seal as in step (a). The process is completed when the last fuel cell is formed, wherein it is then topped off with a bus bar, insulator plate, and the final end plate.
In view of the above, it can be appreciated that each seal has to be carefully placed, and the installer has to ensure that each seal is fully and properly engaged in its sealing groove. It is not uncommon for installers to overlook small portions of seals that may not be properly located. Thus, the seal between adjacent pairs of fuel cells used for the coolant area may have a groove provided in the facing surfaces of the two flow field plates. Necessarily, installers can typically only locate the seal in one of these grooves, and must rely on feel or the like to ensure that the seal properly engages in the groove of the other plate during assembly. In this regard, it is practically impossible for an installer to visually inspect the seal to ensure that it is properly seated in both grooves.
It is possible to mold the seals directly onto the individual cells, as noted above, and this offers advantages during assembly compared to the use of a floppy gasket seal, such as better tolerance and improved part allocation, and can be further improved by techniques according to the present invention.
Thus, it will be appreciated that assembling a conventional fuel cell stack is difficult, time consuming, and can often lead to seal failure. After a complete fuel cell stack has been assembled, it is of course tested, but this procedure can be difficult and complex. Even if leaks are detected, this may initially present itself simply as inability of the stack to maintain pressure of a particular fluid, and it is often difficult to locate exactly where the leak is occurring particularly when the leak is internal. Nevertheless, the only way to repair the fuel cell stack is to disassemble it entirely and replace the faulty seal. This results in a disruption of the other seals so that the entire stack and all the different seals have to be reassembled, which again makes possible misalignment and failure of any one of the numerous seals in the fuel cell stack.
A further problem with conventional assembly and sealing techniques is that the clamping pressure applied to the entire fuel cell stack is intended to serve two different and distinct functions. These functions are (i) to provide a sufficient pressure to ensure that the seals function as intended, and (ii) to provide a desired pressure or compression to the gas diffusion media which is sandwiched between the MEA and the individual flow field plates. If insufficient pressure is applied to the GDM, then poor electrical contact is made; while on the other hand, if the GDM is over compressed, the flow of gas can be compromised.
Unfortunately, in many conventional designs, it is only possible to apply a known total pressure to the overall fuel cell stack. Thus, there is no way of knowing how this pressure is divided between pressure applied to the seals and pressure applied to the GDM. In an otherwise conventional design, this split in applied pressure depends upon design of individual elements in the fuel cell stack, and the maintenance of appropriate tolerances. For example, GDMs commonly lie in center portions of flow field plates, and if the depth of each center portion varies outside an acceptable tolerance, then this results in incorrect pressure being applied to the GDM. The depth can be dependent upon the extent gaskets are compressed which affects their sealing properties, durability, and the lifetime of the seals.
For all these reasons, manufacture and assembly of conventional fuel cells is a time consuming and expensive task, and existing assembly techniques are not suited to any large scale production of a fuel cell on a production line basis. Thus, fuel cell technology is now under development for commercial and residential market application as an alternative power supply worldwide. One area of concern in developing this technology has been the sealing obtained between individual of the fuel cell plates. The materials selected for use in such commercial and residential market applications must meet various manufacturing and functional requirements specific to the applications. Typically, about 2-200 fuel cell plates are being stacked together with silicone rubber seals separating various fuel cell plates, and these fuel cell plates are being constructed of brittle materials which can easily fail due to stress concentrations generated within state of the art groove profiles.
Accordingly, the present invention is directed to circular or elliptical seal groove profiles in fuel cell plates which were developed to minimize stress concentrations in fuel cells and fuel cell stacks, especially for commercial and residential market applications. It has been unexpectedly discovered that these groove profiles enhance the mechanical strength of fuel cell plates over state of the art groove profiles of square or rectangular configuration.
Insert injection molding is the technique used to form the seals in the fuel cell plates. The advantage obtained by using the insert injection molding technique is that the resulting insert injection molded seals can be bonded in the fuel cell plate groove, allowing the remaining seal free surface to be of any shape dependent upon the specific sealing market application. Thus, insert injection molded gasket seals can be bonded in the circular or elliptical groove profile, enabling easier fuel cell plate handling and faster fuel cell stack assembly. The combination of an improved groove profile, and the use of a bonded silicone rubber seal, is considered as providing a significant improvement in fuel cell plate and fuel cell stack design.
In accordance with one aspect of the invention, there is provided a fuel cell assembly comprising (i) a plurality of separate elements; (ii) grooves or a network of grooves extended throughout the fuel cell assembly; and (iii) a seal or gasket within each groove formed by insert injection molding, and wherein the seal and the corresponding groove have profiles of a particular shape such as to provide better sealing between two or more of the separate elements used to define chambers containing fluids used in operating fuel cells and fuel cell stacks. Seal compositions used for forming the seal or gaskets according to the invention are set forth in detail below. These seal compositions are suitable for use at temperatures in the range of xe2x88x9255 to 250xc2x0 C. For purposes of the present invention, seals formed in a fuel cell or other electrochemical cell are referred to as being a unitary seal construction.
The composition of the sealing material used for forming the seal preferably comprises a linear polysiloxane polymer with either terminal or pendant unsaturated organic groups such as the vinyl group xe2x80x94CHxe2x95x90CH2. The polysiloxane can be a polydimethylsiloxane homopolymer, a polymethyltrifluoropropylsiloxane homopolymer, a polydimethyl(methyltrifluoropropyl)siloxane copolymer, or a mixture thereof. The content of methyltrifluoropropyl can be adjusted to provide increased robustness and chemical resistance when mild or aggressive hydrocarbon based coolants are used in operating the fuel cell. The composition may contain other additives dependent upon the specific polymer composition, such as extending fillers, cure systems of platinum silicon hydrides or peroxides, thermal and acid scavengers such as metal oxides or metal hydroxides, as well as adhesion promoters which can be added to meet any of the unique requirements of a fuel cell construction and their requirements for an operation with a long life.
Fuel cells and fuel cell stacks of the present invention provide a number of advantages over conventional fuel cell and fuel cell stack constructions employing separate gaskets. Firstly, the stress in the fuel cell plate and the elastomeric seal is optimized to the point of providing better sealing properties and a longer life. Secondly, the assembly of the fuel cell stack is greatly simplified. Thirdly, tolerance requirements for the grooves for seals can be relaxed considerably, since it is no longer necessary for the seals to correspond to a chosen gasket dimension. Thus, liquid materials used to form the seals can be injected and compensate for a wide range of variations in the profile of the groove and its dimensions.
The use of a liquid sealing material capable of being cured to form an elastomeric sealing material allows the use of compositions designed to chemically bond to various materials used in the construction of fuel cell plates and the other elements of fuel cell stacks, and this ensures and enhances the performance of the seals. It should also increase the overall durability of fuel cell stacks. When it is anticipated that the fuel cell stack design will include the use of an aggressive coolant such as a glycol, with the present invention, it is a simple matter to select a sealing material that is compatible with the particular coolant or other fluids present.
A related advantage is that a more economic construction can be provided. Thus, as noted, it is not necessary for the grooves to be formed to an accurate dimension. Additionally, there is no waste of the gasket material as commonly occurs when cutting gaskets from a sheet material.
Another aspect of the present invention is the provision of a liquid sealing material containing:
(a) 100 parts by weight of a polydiorganosiloxane containing two or more silicon bonded alkenyl groups in its molecule;
(b) 5-50 parts by weight of a reinforcing filler;
(c) 1-20 parts by weight of an oxide or an hydroxide of an alkaline earth metal with an atomic weight of 40 or more;
(d) an organohydrogenpolysiloxane containing three or more silicon bonded hydrogen atoms in its molecule, being present in an amount to provide a molar ratio of silicon bonded hydrogen atoms in the organohydrogenpolysiloxane to silicon bonded alkenyl groups in polydiorganosiloxane (a) of 0.4:1 to 5:1;
(e) a platinum metal type catalyst being present in an amount to provide 0.1-500 parts by weight of platinum metal per one million parts by weight of polydiorganosiloxane (a);
(f) optionally, 0.1-5.0 parts by weight of an organic peroxide being present with or without the presence of ingredient (e);
(g) optionally, 0.01-5.0 parts by weight of an inhibitor;
(h) optionally, 0-100 parts by weight of a non-reinforcing extending filler; and
(i) optionally, 0.1-5.0 parts by weight of an adhesion promoter.
In addition to being applicable to fuel cells, the present invention is generally applicable to electrochemical cells. These and other features of the invention will become apparent from a consideration of the detailed description.